The effect of long-range order on single-crystal elastic constants of Pd_(0.5)Ag_(0.5) alloy has been investigated using first-principles electronic structure calculations. The lowest energy among the considered ordered, partially ordered, and disordered structures is found to be the L11 layered structure, which is formed by alternate (111) Pd and Ag layers. The ordering effect is found to follow a clear trend: in contrast to the disordered phase, for which the K_a and K_c compressibilities are equal, the LI_1 structure becomes less compressible along the c axis than along the a axis.
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